Simultaneous multi-slice (SMS) is an acceleration technique in which several slices are excited and acquired simultaneously leading to a k-space dataset which comprising several slices collapsed on top of each other. Separating or uncollapsing these slices is performed during image reconstruction with the slice GeneRalized Autocalibrating Partial Parallel Acquisition (GRAPPA) method. Excitation, refocusing or magnetization preparation (mag-prep) for these simultaneously acquired slices is performed using a multi-band radio frequency (RF) pulse, as shown in FIG. 2A. A multi-band RF pulse is generated by superposition of the individual single band RF pulses which are typically used to excite/refocus/mag-prep a single slice in conventional MRI. Using a multi band RF pulse ensures that imaging parameters like echo time (TE), inversion time (TI) etc. are identical for all the simultaneously acquired slices.
One drawback of using a multi-band RF pulse is the increase in peak RF power which increases by a factor equal to the number of simultaneously acquired slices (due to superposition as shown in FIG. 2A). The higher peak power requirement is typically not a problem for excitation pulses, but can become a prohibiting factor for refocusing or magnetization preparation (saturation recovery, inversion recovery, Driven Equilibrium Fourier Transform (DEFT) magnetization restore etc.) pulses.
Various techniques have been proposed for addressing the problem of peak RF power for SMS imaging including VErsatile Reaction SEparation (VERSE), the use of phase scrambling during imaging, and the use of time-shifted multi-band pulses during imaging. Each of these techniques has significant drawbacks that limit their general applicability to clinical imaging scenarios. For example, the VERSE technique leads to distorted slice profiles for off-resonance spins. Moreover, each of these techniques still requires a peak power value which is higher than that needed for a single band RF pulse. This can still pose a problem for high power pulses like adiabatic inversion with a hyperbolic secant pulse, or adiabatic saturation pulses, or DEFT restore pulses used in SMS TSE imaging etc.